Structure and luminescent properties of coordination polymers containing lead(II) and thiophene ligands

Three new metal-organic framework polymers [Pb₂(btdc)₂] (1), [Pb₃(ttdc)₃(dma)₂] (2), and [Pb(ttdc)(dma)] (3) (H₂btdc = 2,2′-bithiophene-5,5′-dicarboxylic acid, H₂ttdc = thieno[3,2-b]thiophene-2,5-dicarboxylic acid, dma = N,N-dimethylacetamide) are synthesized under solvatothermal conditions. The compounds are characterized by single crystal and powder X-ray diffraction, elemental, thermogravimetric, and IR spectroscopic analyses. All three compounds are three-dimensional coordination polymers with layered (1) or chain (2, 3) motifs. The study of luminescent properties reveals the intraligand nature of radiative transitions. For 1 luminescence quenching is observed because of the strengthening of vibrational states and long-range interactions between π-systems of thiophene groups. The luminescence spectrum of 2 contains two bands corresponding to two types of ttdc^2– ligands with principally different structural environments. For the ligands isolated from each other the position of the luminescence maximum corresponds to that of the free ttdc^2– ion. For the ligands associated in closely packed chains by continuous π-stacking the appearance of a new maximum in the red range is observed, which is due to the delocalization of electron orbitals.